Airfoils of turbine blades and vanes generally may have a number of cooling holes therein to provide a flow of cooling air to the exterior surfaces of the airfoil and the like. Due to the severe temperatures and conditions in which the turbine airfoils generally operate, protective coatings are often applied to the airfoil and related components after manufacture. Various types of protective coatings may be known. These protective coatings generally are sprayed onto the airfoil and the related components.
One issue with the application is such protective coatings, however, is that the spray may plug one or more of the cooling holes. In order to avoid such, various types of masks and the like may be used to cover the cooling holes during the application of the spray coating. These masks, however, may be difficult and time consuming to apply and remove. Other known practices include the use of a counterbore around at least the opening of the cooling holes so as to act as a “coating collector”, i.e., the spray may accumulate within the counterbore but leave a main passage through the cooling hole open for the cooling air. Although these coating collectors may be effective, typical counterbore designs may break into the casting cavity as airfoil walls become increasingly thinner.
There is thus a desire for an improved airfoil design with cooling holes therein. Preferably, such an airfoil design may provide cooling holes that can accommodate the application of a protective spray coat with increasingly thinner airfoil walls.